In the known quantum key distribution (QKD), a transmission device and a reception device that are connected with an optical fiber use a single photon transmitted continuously between the devices to securely share a cryptographic key. The principle of the quantum mechanics guarantees that the shared cryptographic key based on the quantum key distribution technique has not been eavesdropped. The shared cryptographic key is used in the cryptographic communication such as a one-time pad. The one-time pad is the encrypted communication method in which the cryptographic key with the same size as the data to be exchanged is used and the cryptographic key is discarded after every use. The information theory guarantees that the cryptographic data exchanged by the one-time pad can never be cracked by any skilled eavesdropper.
In the conventional technique, however, it has been difficult to stabilize the distribution speed of the cryptographic key information when the quantum communication system shares the quantum communication channel.